This invention relates to diene based, epoxidized star polymers suitable for crosslinking and adhesive, sealant and coating compositions made therefrom.
High molecular weight diene based styrenic block copolymers such as the family of KRATON.RTM. S-I-S, S-B-S, S-EP-S and S-EB-S block copolymers are extensively used as base polymers in the formulation of many types of hot melt applied coatings and adhesives. These materials offer advantages over other types of materials such as acrylic oligomer/acrylic monomer and polyol/isocyanate monomer systems that are applied as 100% reactive systems and are cured by chemical or radiation means. An important advantage of the high molecular weight diene based styrenic block copolymers is the ability to provide non-polluting formulations and method of application that employs very benign raw materials, compared to the substantial risk associated with acrylate and isocyanate monomer based 100% reactive systems.
However, an important limitation of the styrenic block copolymers has been the inability to provide formulations with solvent and higher temperature resistance. This is because the styrenic block copolymers rely on physical association of the polystyrene blocks in the polymer for crosslinking and once the polystyrene domains in the polymer are weakened by solvent takeup or temperatures near or above the glass transition temperature of the polystyrene, cohesive strength is lost. This problem has been solved by the preparation of very high molecular weight block copolymers such as KRATON.RTM. D1320X rubber which can undergo covalent crosslinking by EB radiation to reinforce the physical crosslinking to give improved solvent and heat resistance. Another important limitation of the styrenic block copolymers is related to the otherwise very desirable hot melt application method. Many substrates cannot tolerate being exposed to hot melt temperatures, 300.degree. F. to 450.degree. F., without being severely damaged. This is the case for very thin polyolefin films, such as thin polyethylene film.
The present invention provides relatively high molecular weight diene based polymers having a very compact star structure with short arms which lowers the viscosity. These polymers do not have a distinct styrene block which would tend to raise application viscosity. The polymers of this invention have many of the property advantages of conventional styrenic block copolymers, including being safe to handle and being covalently crosslinkable for solvent and high temperature resistance. Additionally, the polymers of this invention can be applied as warm melts or as a 100% reactive so that they can be used on heat sensitive substrates. They are sprayable as well. Further, they cure readily, especially by UV radiation.